1. Field of the Invention
The present invention relates to a sheet processing apparatus which selectively subjects sheets to binding and folding processes. More specifically, the present invention relates to a sheet processing apparatus used for an image forming apparatus such as a copying machine, a printer, or a multiple function processing machine having their functions.
2. Description of the Related Art
There have been proposed various sheet processing apparatuses which selectively subject a bundle of a plurality of sheets having an image formed thereon to binding and folding processes. As disclosed in Japanese Patent Application Laid-Open No. 11-193175, there has been known the sheet processing apparatus which makes a bundle of a plurality of sheets to bind its center portion and then folds the bundle into two along its binding portion to make a book bundle.
The sheet processing apparatus described in Japanese Patent Application Laid-Open No. 11-193175 conveys each sheet between a stapler dividedly arranged via a sheet conveying path and then aligns and stacks the sheets to make a sheet bundle. The sheet conveying operation is performed by conveying rollers arranged upstream and downstream of the stapler in the conveying direction of the sheet. The sheet conveyed by the conveying rollers is positioned by abutting its sheet downstream end on an end stopper. The position of the end stopper is set such that the center portion in the conveying direction of the sheet is in the binding position of the stapler for the next binding process. The sheet processing apparatus described in Japanese Patent Application Laid-Open No. 11-193175 moves the position of the end stopper in the conveying direction with reference to the positions of the stapler (or the binding positions) according to the size (or the length in the conveying direction) of the sheet. The aligned sheet bundle can be directly subjected to the binding process without being moved (or conveyed) for the binding process.
In the sheet processing apparatus described in Japanese Patent Application Laid-Open No. 11-193175, the conveying rollers on the upstream and downstream of the stapler need a conveying force which conveys a sheet into between the stapler to reliably abut its downstream end on the end stopper.
When the conveying force is excessively increased, the sheet is buckled between the conveying roller on the downstream and the end stopper, resulting in alignment failure. When the previous sheet is greatly buckled, the next sheet cannot be accepted, which can cause sheet jamming.
The sheet buckling is caused by excessive sheet warping between the conveying roller on the downstream and the end stopper. This can be easily caused as the distance between the conveying roller on the downstream and the end stopper is increased. A sheet which is longer in the conveying direction can be easily buckled.
To prevent the sheet buckling, it is considered a configuration which uniformly brings the end stopper close to the conveying roller on the downstream to a position where a sheet is not buckled regardless of the length in the conveying direction of the sheet. A sheet bundle need to be conveyed a long distance to the binding position for the next binding process depending on the length in the conveying direction of the sheet. Time required for processing the sheet can be increased.